1. The equation for slip speed is ____________
a) (ω-ωr)/ω
b) ω-ωr
c) (ωr-ω)/ω
d) ωr
Explanation: The relative speed between rotor and rotating flux wave, i.e, (ω-ωr) is referred to as the slip speed in rad/sec
slip speed = (ω-ωr) rad/sec.
2. If the rotor of an induction motor is made to revolve in a direction opposite to the rotating flux wave, then RMS value of EMF induced in one phase of rotor E is proportional to ____________
a) 2-s
b) s
c) 1-s
d) 2+s
Explanation: Relative velocity between rotor winding and rotating flux wave becomes
(ω+ωr) = ω+ω(1-s) = ω(2-s) and
E = 2πf(2-s)NphrkwΦ.
3. In an alternator, frequency per revolution is equal to ___________
a) number of poles
b) twice the number of poles
c) speed in rps
d) number of pole-pairs
Explanation: For a P-pole machine, in n rev/sec, P/2n cycles/second are generated and cycles per second is referred to as frequency f of the EMF wave. Here in this question n=1
⇒ frequency per revolution = P/2
⇒ number of pole pairs.
4. The EMF generated in an alternator depends upon which of the following statements are correct?
(i) speed
(ii) maximum flux per pole
(iii) series turns per phase
(iv) phase spread
(v) coil-span
(vi) type of alternator
a) (i), (ii), (iii), (iv), (v), (vi)
b) (i), (iii), (iv), (v)
c) (i), (ii), (iii), (v), (vi)
d) (i), (iii), (iv), (v), (vi)
Explanation: Eph = √2πfkwNphΦ, here Φ = total flux per pole.
5. If the dimensions of all the parts of a synchronous generator and the number of field and armature turns are doubled, then the generated voltage will change by a factor of ________
a) 1
b) 2
c) 4
d) 8
Explanation: E = √2πfrNΦ, Φ = flux per pole = 4/pBplr
dimensions are doubled ⇒ l1 = 2l and r1 = 2r ⇒ Φ1 = 4Φ
No. of turns doubled ⇒ N1 = 2N, fr changes only when the poles are changed ⇒ fr1 = fr
⇒ E1=√2πfr(2N)(4Φ) = 8E
The generated voltage will change by a factor of 8.
6. A 6 pole machine is rotating at a speed of 1200rpm. This speed in mechanical rad/sec and electrical radians per second is respectively?
a) 40π, 40π/3
b) 120π, 40π
c) 20π, 60π
d) 40π, 120π
Explanation: N = 1200rpm,P = 6 and f = PN/120 = 60 cycles per sec.
1cycle = 360° electrical degrees = 2π electrical radians
f in electrical radians per sec = 60∗2π electrical radians/sec = 120π elect radians/sec
speed in mechanical radians/sec = 2/P(speed in electrical rad/sec) = 2/P(120π)=40π mechanical rad/sec.
7. The short pitch winding for an alternator gives which of the following statements?
(i) improved wave form of the generated EMF per phase
(ii) reduced value of self inductance of the winding
(iii) increased rating of machine
(iv) reduced tooth ripples
(v) increased total generated EMF
(vi) saving in winding copper
a) (i), (ii), (iv), (v), (vi)
b) (i), (ii), (vi)
c) (i), (ii), (iv), (vi)
d) (i), (iii), (vi), (iv)
Explanation: The effect of short pitched coil is to reduce the generated EMF, and it improves waveform of EMF generated per phase, and as the windings are placed closed to each other, copper will be saved.
8. The DC machines are designed with flat topped flux density waves because _______________
a) average value of brush voltage is more for a flat topped B-wave
b) average value of brush voltage is less for a flat topped B-wave
c) losses for flat topped B-wave are less
d) none of the mentioned
Explanation: For the same value of peak flux density Bp, it is easy to see that average value of brush voltage could be more for a flat topped B-wave than for a sinusoidal flux density wave.
9. For the given traction application using a dc series motor for a starting time ’t’ is applied. If this method is replaced by a series-parallel control, giving 50% time for each series and parallel. The saving in the starting energy of _________
a) 25%
b) zero
c) 50%
d) 75%
Explanation: Initially let the energy utilized was 100% for time t.Then for 50% duration for series, energy used is 50%. For parallel operation for the same circuit, the resistance gets halved assuming the same machine. So, for 50% of the parallel operation, 25% of energy is only utilized. Hence total saving is 25%.
10. If two series motors are identically coupled. One is running as motor while other as generator. For this combination, the iron losses and frictional losses are found identical for _________
a) identical speed and excitation
b) identical speed
c) identical rating and construction
d) none of the mentioned
Explanation: For same iron and frictional losses, same frequency should be there. For that same speed must be run.